Coaxial multipactor switch utilizing magnetic field to control impedance



March 14, 1967 J. F. KANE 3,309,561 COAXIAL MULTIPACTOR SWITCH UTILIZINGMAGNETIC FIELD TO CONTROL IMPEDANCE Filed Jan. 31, 1963 UTILIZATICNDEVICE DIELECTRIC T? INVENTOR 'JOHN E KANE ATTORNEY United States Patent3,309,561 COAXIAL MULTIPACTQR SWITCH UTILIZING MAGNETIC FIELD TO CONTROLIMPEDANCE John Frederick Kane, Mountain View, Calif., assignor toGeneral Electric Company, a corporation of New York Filerl Jan. 31,1963, Ser. No. 255,362 5 Claims. (Cl. 315-39) prohibit the passage ofmicrowaves through a particular waveguide is known as a waveguideswitch.

Prior art variable impedances and switches for microwave transmissionand control include mechanical, gaseous, and ferrite devices. In themechanical devices shutters, plungers, or vanes are selectivelypositioned within the waveguide to control microwave transmission; i.e.,one form of waveguide switch comprises a plate rotatable for closing thewaveguide passage to prohibit transmission.

In the gaseous devices the microwaves are transmitted throughgas-enclosing cells disposed within the waveguide, and a high voltage isapplied across the gas cell to produce ionization of the gas moleculesand thereby inhibit trans mission of microwaves through the cell. In theferrite devices, various configurations of ferrite material are disposedwithin the waveguide, and controllable magnetic fields are applied tothe ferrite material to thereby prohibit, permit, or restrictivelypermit the transmission of microwaves through the waveguide.

A common disadvantage of the prior art mechanical and gaseous variableimpedances is their relative slowness of operation. In modern microwavesystems, it is common to transmit microwaves in bursts, known as pulses,spaced but a few millionths of a sec-0nd (microseconds) apart, theduration of the bursts frequently being substantially less thanone-millionth of a second. In such systems it is desirable to be able toalter the value of a microwave impedance in the interval between twosuccessive pulses. However, in the mechanical type variable impedancesand switches the inertia of the structure normally prohibits effectivecontrol, or change in operation, in less than one- -thousandth of asecond. In the gaseous variable impedanoes, once the gas has becomeionized many microseconds are required for the gas to deionize so as toonce again permit unimpeded passage of the microwaves therethrough. Thisgaseous type of variable impedance is further characterized by arelatively short'life and by the generation of undesirable noise whenmicrowaves are transmitted therethrough in the presence of residual gasions. A disadvantages of the ferrite-type variable impedances is thepresence of a permanent obstacle of ferrite material within the hollowwaveguide. The ferrite obstacle induces reflections in the waveguidewhen microwaves are transmitted therethrough and also limits the powerhandling capabilities of the waveguide. For example, in one form offerrite variable impedance relatively large microwave powers would tendto destroy the ferrite material. i p

Therefore, in modern microwave technology, it is desirable to provide arapidly variable impedance or switch that functions as an unimpededtransmission path in the ice intervals between two successive pulses,which may be spaced but a few microseconds apart. Additionally, it isdesirable that such a variable impedance or switch control the highestpossible levels of microwave power.

Accordingly, it is the principal object of this invention to provide animproved microwave impedance.

Another object of this invention is to provide animproved microwaveswitch.

Another object of this invention is to provide a microwave device whoseimpedance may be varied at rapid rates.

Another object of this invention is to provide a microwave device whoseimpedance may be varied at rapid rates and which is adapted to operateat very high levels of microwave power.

The foregoing objects are achieved, according to one form of theinvention, by providing a hollow coaxial transmission line havingconcentrically disposed inner and outer conductors for transmittingmicrowaves therethrough and by providing a variable impedance in theform of a con trollable multipactor discharge between such conductors. Asection of the transmission line is evacuated and sealed to maintain aregion between the conductors evacuated. The opposing surfaces of thetwo conductors comprise a material characterized by a secondary emissionratio greater than unity for the energy level of the electromagneticwaves supplied. The relative diameters of the two conductors are chosento support a multipactor discharge therebetween. A multipactor dischargeis a sustained secondary emission discharge between opposing surfaces asa result of the motion of secondary electrons produced at the surfacesin synchronism with the alternating field of electromagnetic energyexisting between the surfaces. A controlling magnetic field of variablestrength is applied to the evacuated section of the transmission line,the field being oriented in a direction parallel to the axis of thecoaxial line. In operation, when the magnetic field strength has aminimum value the multipactor discharge has substantially maximumstrength in the presence of the electromagnetic energy, the dischargethereby simulating a short circuit or relatively low impedance betweenthe conductors. When the magnetic field strength is increased to apredetermined value, the multipactor discharge is rendered less intenseor is extinguished, thereby permitting the passage of electromagneticwaves. A rapid application of the magnetic field provides an equallyrapid extinction of the multipactor discharge, whereas a suddenreduction of the magnetic. field provides an equally rapid initiation ofthe multipactor discharge. Thus, a rapidly responding controllableimpedance is provided by the unimpeded coaxial line with controllablemultipactor discharge, which also provides for transmission of maximumlevels of microwave power.

The invention will be described with reference to the accompanyingdrawing wherein:

FIGURE 1 is a perspective view, partly in cross-section, of one form ofthe invention; and

FIGURES 2a and 2b are partial cross-sectional views of the device ofFIGURE 1, illustrating the principle of opi eration thereof.

The variable impedance cell 10 of FIG. 1 is illustrated as providing avariable impedance or switching function between a transmitter 11 ofmicrowaves and a utilization devicp 12. Transmitter 11 is of the typeprovided for delivering electromagnetic waves with high levels of energyduring very short periods, these periods of transmission being known aspulses. However, it is within the scope of the instant invention fortransmitter 11 to deliver electromagnetic waves continuously.Transmitter 11 is coupled to a microwave transmission line 13, shownsche- 3 matically, for transmitting electromagnetic energy into one endof cell 10.

A utilization device 12, such as a microwave receiver or an antenna,utilizes the microwave energy delivered by cell 10. Utilization device12 is coupled to a transmission line 14, shown schematically, forreceiving the energy transmitted from the other end of cell 10.

Variable impedance cell comprises hollow coaxial transmission line 16having concentrically disposed inner conductor 17 and outer conductor18. A pair of annular dielectric windows 20 and 21 are each afiixed andsealed to the outer surface of inner conductor 17 and tothe opposinginner surface of outer conductor 18. Windows 20 and 21 function as gasseals, transparent to the passage of electromagnetic energy, butimpervious to the passage of gas molecules. The interior of the sectionof coaxial line 16 between windows 20 and 21 is evacuated. Thus windows20 and 21 provide a seal to maintain an evacuated region in the sectionof coaxial line 16 between the two windows, while permitting passage ofelectromagnetic waves through such section.

The evacuated section of coaxial line 16 is adapted for immersion in amagnetic field, B, provided by a source of magnetomotive force, suchmagnetic field being directed parallel to the axis of the coaxial line.This source of magnetomotive force, in one form, comprises an elongatedhollow cylindrical solenoid 23. Solenoid 23 is energized by acontrollable current source 24. Current source 24 is shown schematicallyas being electrically connected to solenoid 23 for providing electriccurrent through the turns of the solenoid to produce the requisitemagnetic field in coaxial line 16.

The manner of operation of the variable impedance cell, as presentlyunderstood, will now be described. The operation will first be describedin the absence of any magnetic field provided by solenoid 23-. In FIG.2a, the dashed radial lines represent, at a given instant, thedistribution and direction of the electric field component of theelectromagnetic energy between the opposed surfaces of conductors 17 and18 at a particular cross-sectional plane. For electromagnetic energy ofmicrowave frequencies, the electric field illustrated reverses itsdirection at thousands of megacycles each second. Thus, at one momentthe radial electric field will be directed outwardly, as illustrated in'FIG. 2a and one half cycle later it will be directed inwardly.

This electric field represents the force on positive electrical charges,and the direction of the electric field repre sents the direction ofsuch force. Thus, at the location and at the moment illustrated in FIG.2a positive charges will experience an outward radial force and,consequently, will be accelerated toward outer conductor 18. Conversely,negative charges at the same moment and location will experience aninward radial force and will be accelerated toward inner conductor 17.

It is well known that the entire region immediately above the earthssurface is continually subjected to radioactive emissions from materialsin the earths surface and from cosmic rays. A significant portion of thematter receiving these radioactive emissions and cosmic rays will beionized thereby; in other words, such matter will be broken intopositive and negative charges, wherein the negative charges areelectrons. Therefore, the interior of the evacuated region of coaxialline 16 is always subjected to these radioactive emissions and cosmicrays. Since a perfect vacuum can never be obtained, this evacuatedregion always contains a small number of residual gas molecules.Consequently, ionization of a small fraction of these residual gasmolecules will continually be occurring, so that a number of electronsare always present in the evacuated region of coaxial line 16. Theseelectrons will be accelerated by the electric field of the microwaveenergy when transmitter 11 generates pulses and many electrons willstrike one of conductors 17 and 18.

When electrons strike a surface, other electrons are driven from thesurface by the energy of impact. The electrons striking the surface aretermed primary electrons and the electrons driven from the surface aretermed secondary electrons. The ratio of the number of secondaryelectrons created to the number of primary electrons striking a surfaceis defined as the secondary-emission ratio. The secondary-emission ratiovaries with the velocity of the primary electrons and the material ofthe surface. For many types of surface materials secondaryemissionratios greater than unity are provided for a Wide range of electronvelocities; for example, see K. R. Spangen=berg, Vacuum Tubes, pages4857, McGraW-Hill Book Company, Inc., New York, 1948. However, thesesurface materials do not provide a secondary-emission ratio greater thanunity for very slow or very fast primary electrons. The outer surface ofinner conductor 17 and the inner surface of outer conductor 17 in theevacuated region are provided with such a surface material adapted toyield a secondary-emission ratio greater than unity for primaryelectrons having a wide range of velocities. One such surface materialparticularly suitable for employment for this purpose is an alloy ofsilver and magnesium, such as that described by V. K. Zworykin, J. E.Ruedy, and E. W. Pike, Silver-Magnesium Alloy as a Secondary ElectronEmitting Material, J. Appl. Phys., vol. 12, pages 696698, September1941.

Consequently, if the strength of the electric field component of theelectromagnetic energy in the evacuated regions between conductors 17and 18 is sufliciently great, a number of the electrons formed in theevacuated portion will be accelerated to strike the opposing surfaces ofconductors 17 and 18 with sufficient energy to create a greater numberof secondary electrons. Providing that the electric field reversesdirection immediately after creation of the secondary electrons theywill be accelerated in a direction away from the surface at which theywere created and toward the opposing conductor surface. Thus, considerthe instant depicted in FIG. 2a. Electrons created in the evacuatedregion of coaxial line 16 will be accelerated toward inner conductor 17.If the electric field persists in the direction shown sufficiently longfor these accelerated electrons to strike conductor 17, secondaryelectrons will be emitted therefrom greater in number than the primaryelectrons, provided the electric field strength is sufficiently great.If, now, the electric field reverses its direction immediately aftercreation of the secondary electrons, they will be accelerated towardouter conductor 18. Once again,if the electric field pro vided in thisreverse direction persists sufificiently long for this group ofsecondary electrons to travel from inner conductor 17 to outer conductor18, a new group of sec ondary electrons will be impelled from the innersurface of outer conductor 18, this new group being greater in numberthan the original group of secondary electrons striking conductor 18.Again, if the electric field reverses immediately after formation ofthis new group of sec ondary electrons, the new group will beaccelerated to- Ward inner conductor 17. Thus, with a sufficient electrical field strength and an appropriate frequency of theelectromagnetic energy, secondary electron groups travel back-and-forthbetween the opposed surfaces of conductors 17 and 18. With suchfavorable conditions the size of each secondary electron group growsafter each reversal until a sheet of electron current is created acrossthe evacuated regions between the opposed surfaces of conductors 17 and18.

The current flowing in the manner described is termed a multipactordischarge and, therefore, is defined as a sustained secondary-emissiondischarge existing between the conductors as a result of the motion ofsecondary electrons in synchronism with a strong rapidly alternatingelectric field applied to the region. Accordingly, the relativediameters of coaxial line 16 are adjusted by design or experiment sothat synchronism of the secondary electrons with electromagnetic energyof appropriate strength and frequency provides a multipactor discharge.Tr-ajectory 28 illustrates a path followed by the secondary electrongroups in the multipactor discharge when no magnetic field is providedby solenoid 23. Such paths generally will occur along radialtrajectories at all angles about the coaxial line axis in FIG. 2a.

As described thus far, a multipactor discharge occurs between conductors17 and 18 in" the evacuated region of coaxial line 16 whenelectromagn'eticenergy of sufficient intensity is received therein,provided that source 24 is adjusted so that no magnetic field issupplied by solenoid 23. This multipactor discharge willfunction as anequivalent short circuit between conductors 17 and 18, therebyattenuating, or preventing, transmission of electromagnetic energybetween transmitter 11 and utilization device 12. If electromagneticenergy is provided as pulses by transmitter 11, the multipactordischarge will take place in coaxial line 16 only during the occurrenceof these pulses.

Consider, now, the operation of the 'cell of FIG. 1 when the value ofcurrent delivered by source 24 is increased to increase the strength ofthe axial magnetic field, B, applied to the evacuated region of coaxialline 16. In FIG. 2b this field is directed perpendicularly to thesurface of and enters the cross-section shown. Whena charged particlemoves through a magnetic field, it is subjected to a force orientedperpendicularly to both the direction of particle motion and thedirection of the magnetic field vector. Therefore, an electron travelingfrom outer conductor 18 toward inner conductor 17 experiences a forcetending to bend the path of the electron toward the right of thedirection of motion, as shown in FIG. 2b. If the intensity of themagnetic field is increased to a sufficiently large value, thepreponderance of the secondary electrons emitted from the interiorsurface of outer conductor 18 will be bent sufiiciently, as alongtrajectory 29, so as to miss inner conductor .17. With the applicationof a magnetic field of such intensity a group of secondary electronsemitted from outer conductor 18 will not generate a new and larger groupof secondary electrons at inner conductor 17, so that the conditions fora multipactor discharge are not present.

Therefore, as shown in FIG. 2b, in the presence of an axial magneticfield of substantial intensity a multipactor discharge cannot beinitiated. On the other hand, if a multipactor discharge has beeninduced in the absence of a magnetic field and the magnetic field issubsequently applied, the multipactor discharge will be quenched.Conversely, in the presence of electromagnetic fields of sufficientintensity, the removal of a strong axial magnetic field will permit themultipactor discharge to be initiated in the evacuated region.Accordingly, a controllable multipactor discharge is provided byapplying a controllable magnetic field to the evacuated region of thecell of FIG. 1. By rapidly varying the intensity of the magnetic fieldthe multipactor discharge may be rapidly initiated or quenched.Additionally, the intensity of the multipactor discharge may be variedby gradually increasing or decreasing the magnetic field so that avariable number of the secondary electrons created at outer conductor 18strike inner conductor 17.

Although the simple trajectory 29 of FIG. 2b may be modified somewhat bythe presence of the alternating electric field of the electromagneticwaves, nevertheless, the trajectory will be essentially as described andthe theory of operation described above is essentially unchanged. Thecontrollable magnetic field applied to the cell of FIG/1 will provide amultipactor discharge of variable intensity and impedance.

While the principles of the invention have now been made clear in anillustrative embodiment, there will be immediately obvious to thoseskilled in the art many modifications in structure, arrangement,proportions, the elements, materials, and components used in thepractice 6. of the invent-ion, and otherwise, which are particularlyadapted for specific environments and operating requirements, withoutdeparting from those principles. The appended claims are thereforeintended to cover and embrace any such modifications, within the limitsonly of the true spirit and scope of the invention.

What is claimed is: 1. A microwave transmission line system comprising avariable impedance cell located between a source and a load, said cellfor providing a variable impedance comprisingi a conductive wallpartially bounding a region, means for maintaining said regionsubstantially evacuated, an electromagnetic wave generator coupled tosupply electromagnetic energy to said region, the level of said energybeing sufficient to induce a multipactor discharge in said region alonga path having at least one end terminating at said wall, and a magneticfield source for applying to said region a steady magnetic field, saidmagnetic field being oriented perpendicularly to said path, saidmultipactor discharge having a maximum value when the magnetic field iszero. 2. The cell of claim 1 further including means for varying thestrength of said magnetic field.

3. A microwave transmission line system comprising a variable impedancecell located between a source and a load, said cell for providing avariable impedance comprising:

a conductive wall partially bounding a region,

means for maintaining said region substantially evacuated,

a controllable magnetic field source for applying to said region asteady magnetic field oriented in a predetermined direction,

an electromagnetic wave generator coupled to supply to said regionelectromagnetic energy having the electric field components thereoforiented perpendicularly to said direction, the level of said energybeing suflicient to induce a multipactor discharge in said region in theabsence of said steady magnetic field, and

means for controlling said field source to apply a magnetic field havingsufficient intensity to quench said multipactor discharge.

4. A Wave transmission device for supporting a controllable multipactordischarge comprising: a coaxial transmission line having concentricallydisposed inner and outer conductors, said conductors comprising amaterial characterized by a secondary-emission ratio than unity forelectromagnetic waves in said section having said energy level, meansfor maintaining a section of said transmission line substantiallyevacuated, an electromagnetic wave generator coupled to transmitelectromagnetic waves to said transmission line at an energy levelsuflicient to induce a multipactor discharge in said section, and amagnetic field source for applying to said section a magnetic fieldoriented parallel to the axis of said transmission line.

5. A Wave transmission device for supporting a controllable multipactordischarge comprising:

a coaxial transmission line having concentrically disposed inner andouter conductors,

means for maintaining a section of said transmission line substantiallyevacuated,

a controllable magnetic field source for applying to said section asteady magnetic field oriented parallel to the axis of said transmissionline,

an electromagnetic wave generator coupled to transmit electromagneticwaves to said transmission line at an energy level sufficient to inducea multipactor discharge in said section between said conductors in theabsence of said steady magnetic field, and

means for controlling said field source to apply a magnetic field havingsufiicient intensity to quench said multipactor discharge.

References Cited by the Examiner 5 UNITED STATES PATENTS 8/1940 Keyston313-104 X 10/1940 George et al 313104 X 2/1951 Linder 313103 X 7/1953Varela 333-451 8 OTHER REFERENCES D. H. Preist and R. C. Talcott: On theHeating of Output Windows of Microwave Tubes by Electron Bombardment;IRE Transactions on Electron Devices, July, 1961 (pp. 243251 relied on).

HERMAN KARL SAALBACH, Primary Examiner.

R. D. COHN, Assistant Examiner.

1. A MICROWAVE TRANSMISSION LINE SYSTEM COMPRISING A VARIABLE IMPEDANCECELL LOCATED BETWEEN A SOURCE AND A LOAD, SAID CELL FOR PROVIDING AVARIABLE IMPEDANCE COMPRISING: A CONDUCTIVE WALL PARTIALLY BOUNDING AREGION, MEANS FOR MAINTAINING SAID REGION SUBSTANTIALLY EVACUATED, ANELECTROMAGNETIC WAVE GENERATOR COUPLED TO SUPPLY ELECTROMAGNETIC ENERGYTO SAID REGION, THE LEVEL OF SAID ENERGY BEING SUFFICIENT TO INDUCE AMULTIPACTOR DISCHARGE IN SAID REGION ALONG A PATH HAVING AT LEAST ONEEND TERMINATING AT SAID WALL, AND A MAGNETIC FIELD SOURCE FOR APPLYINGTO SAID REGION A STEADY MAGNETIC FIELD, SAID MAGNETIC FIELD BEINGORIENTED PERPENDICULARLY TO SAID PATH, SAID MULTIPACTOR DISCHARGE HAVINGA MAXIMUM VALUE WHEN THE MAGNETIC FIELD IS ZERO.